History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
The Corinthians, however, although the others were impatient for the voyage, were not disposed to sail with them until they should have celebrated the Isthmian Games, which were held at that time. And Agis was quite ready to allow them to preserve inviolate the Isthmian truce[*](The Isthmian Games were held in Corinthian territory, and under the presidency of Corinth. They were a τριετηρίς, ie. held every two years, in early spring or summer.) while he made the expedition an affair of his own.
But as the Corinthians did not agree and delay ensued, the Athenians became more aware of the designs of the Chians, and sending Aristocrates, one of their generals, they charged them with the plot, and when they denied it, bade them, as their guarantee of good faith, send some ships along with the Athenian fleet as a contribution to the allied force;
and they sent seven. Their reason for sending these ships was that most of the Chians had no knowledge of the negotiations, and the oligarchs, who were in the plot, were not only unwilling as yet to incur the hostility of the populace, before they had acquired any strength, but also because of the delay of the Peloponnesians no longer expected them to come.
In the meantime the Isthmian Games were celebrated, and the Athenians, since the truce had been proclaimed,[*](During the truce, states which were at war with each other were allowed to send contestants and deputies to the common games, these having a safe pass to and from the celebration.) sent deputies to them; and so the designs of the Chians became more manifest to them. And when they returned they immediately made arrangements that the ships should not set sail from Cenchreiae without their knowledge.
But the Peloponnesians, after the festival, put to sea for Chios with twenty-one ships under the command of Alcamenes. And the Athenians at first sailed up to them with an equal number of ships, and tried to draw them out into the open sea. But when the Peloponnesians did not follow them very far but turned back, the Athenians also withdrew; for they had the seven Chian ships in the ranks of their fleet and did not consider them trustworthy.